Making grid-like elements for electron tubes



May 31, 1966 w. R. STUART MAKING GRID-LIKE ELEMENTS FOR ELECTRON TUBESill! Filed March 20, 1961 INVENTOR. WILLIAM R. STUART ATTORNEY UnitedStates Patent 3,253,620 MAKING GRID-LIKE ELEMENTS FOR ELECTRON TUBESWilliam R. Stuart, San Carlos, Califi, assiguor, by mesne assignments,to Varian Associates, a corporation of California Filed Mar. 20, 1961,Ser. No. 96,841 7 Claims. (Cl. 140-715) This invention relates to theart of making grid-like elements for electron tubes and moreparticularly to a method and apparatus for making extremely accurategrid-like elements.

The term grid-like element as used herein means any element of anelectron tube shaped like a grid, whether it be a heater, cathode, gridelectrode, or some other element. However, for clarity of descriptionthe invention will be explained with particular reference to gridelectrodes. The application of the invention to other gridlike elementswill be obvious to those skilled in the art.

The grid electrode is an element of electron tubes which is largelyresponsible for the high cost of manufacture and for the limits onoperating capabilities of electron tubes.

The more perfect a grid electrode can be made, the closer is can beplaced to adjacent electrodes without shorting, the more advanced can bethe electrical charac teristics of the tube such as higher frequency andthe more uniform will be the operation of each tube of a given class.

In the past the manufacture of grid electrodes has involved a very largeamount of hand labor in manufacturing them and an extremely high degreeof skill in mounting them in tubes. This high degree of mounting skillwas required because grid electrodes were necessarily imperfect becauseof the way in which they were made. The high labor content inmanufacture and the high skill required for mounting are most pronouncedin connection with the manufacture of cylindrical grid electrodes andthe mounting of plural cylindrical grid electrodes coaxially in a tubewith the grid bars of one of the grid electrodes radially and verticallyaligned with the bars of the other grid electrodes. However, it will beobvious that the invention is applicable to the manufacture of othergrid electrode shapes, such as planar grid electrodes and to themounting of single grid electrodes in tubes.

Referring to the pronounced case of the cylindrical grid electrode, theprior method of manufacture has been to wind grid wire on a rod-shapedmandrel so as to form a plurality of parallel wire lengths laying alongthe length of the mandrel in a cylindrical array. The portions of Wireconnecting the lengths together were of course bent over the end of themandrel. A ring-shaped support for the grid electrode was then slideover the mandrel to a position at a predetermined distance from the endof the mandrel. Next each length of grid wire was individually spotwelded to the inside rim of said support ring, a time consum ing andexpensive process.

The grid electrodes made by the prior art technique were imperfect. Itwill be recalled that the lengths of wire had been bent over the end ofthe mandrel, and'as a result they tend to relieve the stress of thesebends by bowing out. To make matters worse, the lengths do not how outuniformly because they pass over the edge of mandrel at different anglesto tangents to the periphery of the edge, some at right angles and someat other angles. Those lengths which passed over the end at rightanglestend to bow straight out while those at other angles tend to bow towardor away from each other. In addition, the lengths tended to bend as aresult of the curved shape they have developed from the shipping spool.In any event the lengths of wire on grid electrodes made prior to thisinvention were neither perfectly straight nor perfectly parallel, andthese imperfections were of a magnitude visible to the naked eye.

Some of the grid electrodes were of course better than others, andconsiderable time and skill was wasted rejecting those which wereunusable. Moreover endless hours were spent by extremely skillfultechnicians trying to match grid electrodes for use in tubes havingplural grid electrodes, and then orient the plural grid electrodes in amanner of best compromise to obtain as nearly as possi ble a conditionwhere each length of wire in each grid electrode is parallel with andradially in line with the corresponding bar in the one or moreadditional grid elec trodes.

It is an object of this invention to provide improved electron tubes byproviding improved grid-like elements for them.

Another object of the invention is to make grid-like ele ments lessexpensive to manufacture and install in elec tron tubes, a furtherobject being to make more accurate grid-like elements.

An additional object of the invention is to provide a new method ofmanufacturing grid-like elements which results in much more accuratestructures.

Another object is to provide a new method of manu facturing grid-likeelements which permits an accurate structure to be made by brazing.

A further object is to provide eflicient apparatus for making grid-likeelements according to the invention.

These and other objects, features and advantages will become apparentfrom the following description when read in conjunction with theaccompanying drawings in which FIGURE 1 is an elevational view of amandrel without any grid wire on it; 7

FIGURE 2 is an elevational view of the mandrel after grid wire has beenwound on it;

FIGURE 3 is an elevational View of the wound mandrel associated withjigging and other elements required for forming a finished gridelectrode according to the invention; and

FIGURE 4 is an elevational view of a finished grid elec trode madeaccording to the invention.

Referring in more detail to the drawings, FIGURE 1 shows a mandrel orforming member 5, having a plurality of parallel grooves 6, a notchedrim 7, and a tying pin 8 projecting from a shaft portion 9. The upperend of the mandrel is preferably slightly recessed so that the landsbetween grooves 6 project above the end as shown in FIGURE 1.

FIGURE 2 shows the mandrel of FIGURE 1 after grid wire has been wound onit. The Wire is wound by tying one end on pin 8, laying the wire in oneof the notches in rim 7 and along one of the grooves 6, over the end ofthe mandrel, down along the mandrel in another groove, through anothernotch in rim 7, across the bottom of rim 7, up through still anothernotch, and so on until all of the grooves 6 are filled. The wire is thentied on pin 8 and finally cut from the spool. This winding techniqueresults in a plurality of lengths of wire 12 running along the mandreland a plurality of connection portions 13 passing over the top of themandrel. Various patterns are available for the connecting portionsdepending on the sequence in which the grooves 6 are filled. Theinvention is not dependent upon selection of any particular pattern forconnecting portions 13.

The apparatus and procedures thus far described are well known in theart. Past practice next involved sliding a supporting member or ring 15over the loaded mandrel. The supporting member or ring 15 is also partof the present invention, and, as in the prior practice, has a collarportion 16 which fits snugly around the grid C; Wire lengths 12.According to the prior technique, each wire length 12 was individuallyspot welded to collar'16 and the lower portions of lengths 12 werebroken off at the welds to make a finished grid electrode.

According to the invention the loaded mandrel is inserted in a jig 17consisting of a base 18 and four upwardly extending legs 19. The twolegs in the foreground are cut away in FIGURE 3 for clarity. The basehas a central bore 20 through which the shaft portion 9 of the mandrelpasses. In practice the base is placed on a flat surface and the loadedmandrel is dropped into place so that the bottom of shaft portion 9 alsorests on the flat surface, and then ring 15 is pushed down into contactwith the upper ends of legs 19. In this manner the position of supportring 15 is accurately determined relative to the top of the mandrel andthe proper length grid electrode is obtained.

The preferred practice of the invention involves brazing the wirelengths 12 to the collar 16. This is accomplished by painting a brazingpaint on wire lengths 12 just above collar 16; or the paint can beapplied to the inner surface of collar 16 before it is placed on themandrel. Another possibility is-to plate the wire or rim 16 with amaterial such as gold which will form a brazing alloy with the basemetals involved, such as copper. In any case it is beneficial to cutaway the lower portions of the lengths 12, as shown in FIGURE 3,.priorto heating the assembly to brazing temperature. In this way the lowerends of the wire lengths are free to expand and contract Without causingbuckling and distortion.

In addition to cutting the lower ends of the wire lengths beforebrazing, a clamp 22 is placed around the wire lengths before brazing.The clamp is made of several segments, preferably four, provided withgrooves 23, around which wires 24 are tightened to hold the segmentspressed against the wire lengths 12. The tension of wires 24 is notcritical but is preferably suflicient to hold the clamp by friction fromsliding down the mandrel by gravity before the brazing step. It shouldbe understood that the grooves 6 are shallow enough relative to thethickness of wire lengths 12 that clamp 22 contacts the wire lengthsrather than the lands between the grooves 6. After the wires have beencut and the clamp 22 has been put in place, the entire apparatus shownin FIGURE 3 is placed in a brazing furnace and collar 16 is brazed towire lengths 12. Next the apparatus is removed from the furnace; theclamp is taken off; the ring 15 and attached wires are removed from themandrel; and finally the lower ends of wires 12 are broken oif' at thebottom edge of collar 16 to form the finished grid.

When materials for the various parts are chosen with the propercoetficients of expansion relative to each other, the clamp 22 will holdwire lengths 12 firmly in place against the mandrel in grooves 6 duringbrazing. The result is that when the assembly is placed in a furnace andheated to brazing temperature, the wire lengths 12 are forced to remainin the perfect shapes provided by the grooves 6, and then when the wirelengths are stress relieved by being heated they are stress relieved inperfect position rather than in the distorted shapes they would tend toassume without clamp 22. The term stress relieved signifies the resultof the process called stress relieving, defined by Metals Handbook,published by the American Society of Metals, as a process of reducingresidual stresses in a metal object by heating the object to a suitabletemperature and holding for a sulficient time.

'There are at least two reasons why a grid electrode formed by theinvention is so superior in shape to those made by the prior art. One isthat according to the prior art, even if each wire length 12 remained inits groove during spot welding, it was not stress relieved becausewelding provides only a very local heating. Thus as soon as the mandrelis removed after spot welding, the wire lengths bowed out of shape as aresult of the internal stresses caused by bending the Wire onto themandrel and the stresses caused by bending the wire onto the carryingspool. Another reason for the relatively poor results of the priorpractice is that not only did it not relieve stresses, it created themdue to the extremely localized heating effect of spot welding. Anincidental but extremely important benefit of the invention is that thebrazed connection between wire lengths 12 and collar 16 has such animproved thermal efiiciency over the prior spot welds that in tubes inwhich the prior grid electrodes operated so hot that'they had to be goldcoated to prevent electron emission, the new grid electrodes run so coolthat the expensive gold coating can be eliminated.

Various materials and temperatures will now be described in more detail.A preferred material for the mandrel '5 is stainless steel having a highchrome content, such as No. .304 stain-less steel made by theInternational Nickel Company. One of the problems with the new processwas the problem of keeping the wire lengths from be coming brazed to themandrel as well as to the support ring 15. This problem was solved byproviding a stable oxide coating on .the mandrel, which in the case ofNo'. 304 steel is chrome oxide obtained by firing in wet hydrogen. Inone application of the principles of the invention the support ring 15is made of copper; the clamp 22 is made of No. 304 steel; the tensionwire 24 is .020 inch diameter molybdenum; the grid wire is .0108 inchdiameter molybdenum; and the brazing material is coppernickel-gold inpaste form. The assembly is heated in an oven to a temperature of about1045 C. for about 10 minutes or more. Uniformly heating the grid wire(while pressed against the mandrel) to a temperature which results inany stress relieving of the grid wire will result in more perfect gridelectrodes than those made by the prior art. However, the optimum resultis obtained by heating the grid wire to recrystallization temperature asdefined in Metals Handbook, published by the American Society forMetals, in the following words: the lowest temperature at which thedistorted grain structure of a cold worked metal is replaced by a newstrain-free grain structure during prolonged annealing. Time, purity ofthe metal, and prior deformation are important factors.

Different materials can be used for the various parts but certainfeatures and relations must [be maintained. For example the mandrel musthave a surface which will not braze to the grid. The relativecoefiicients of expansion of the mandrel and support ring 15 must besuch as to prevent the collar .16 from expanding away from the wirelengths .12 during brazing. The coefiicient of expansion of tension wire24 must be no greater than the mandrel and preferably less so that firmpressure of the clamp 22 against the wire lengths is assured at hightemperature when the grid wire is stress relieved. A different type ofclamp could be used; for example, an apertu-red one-piece clamp could beused if its mandrel receiving aperture were machined to close toleranceand if the coefiicients of expansion were in just the right relation tocause pressure between the clamp and the wire lengths during brazing. Aspreviously mentioned the teachings of the invention are applicable toany grid-like element for an electron tube. For example the inventioncould (be used to make other than cylindrical shapes and have other thanall parallel wires simply by changing the shape of the mandrel and thepattern of its grooves. If in some cases it would be desirable ornecessary to bond the wire lengths .12 to collar 16 first by somelocalized heating process a teachmg of the invention could still beapplied by thereafter heating :the grid Wire uniformly with the clamp 22in.

place.

I claim:

1. A method of making a grid-like element for electron tubes comprisingthe steps of bending wire onto a forming member to make a desired shape,placing a support member in contact with the formed grid Wire,positioning a clamping structure around said forming member in contactwith the formed grid wire, said clamping structure having a lowercoefiicient of expansion than said forming member, and by theapplication of heat simultaneously brazing said support member to theformed grid wire and compressing the wire between the forming member andclamping structure.

2. A method of making a grid-like element for electron tubes comprisingthe steps of placing wire on a forming member, placing a grid support incontact with the wire on the form, bonding said grid support to thewire, heating and cooling said wire uniformly while on said formingmember, and holding said wire in place against said forming memberduring the heating and cooling process, the temperature to which saidwire is heated being substantially the recrystallization temperature ofthe wire.

3. A method of making a grid-like element for electron tubes comprisingthe steps of bending wire across the top and down the sides of a rodshaped mandrel to provide wire lengths extending along the surface ofthe mandrel and fastened to the mandrel below the top of the mandrel,sliding a supporting ring over the mandrel to a selected position abovethe point at which the wires are fastened to the mandrel, cutting saidwire lengths between said fastening point and said supporting ring,placing a clamp around said mandrel above said ring, and brazing saidWire lengths to said supporting member.

4. A method of making a grid-like element for electron tubes comprisingthe steps of placing wire on a forming member to make a desired wireshape, heating and cooling said wire while on said forming member, andholding said wire in place against said forming member during theheating and cooling process, the temperature to which said wire isheated being the recrystallization temperature of the wire.

5. A method of making a grid-like element for electron tubes comprisingthe steps of bending wire onto a forming member to make a desired shape,heating and cooling said wire while on said forming member, and holdingsaid wire in place against said forming member during the heating andcooling process, the temperature to which said wire is heated beingsufficiently high and of sufficient duration to cause said wire to .besubstantially stress relieved.

6. Apparatus for making grid-like elements comprising a forming memberhaving a grid forming surface on which grid wire can be placed in adesired shape, a clamping structure surrounding said forming member andhaving a pressure surface with a shape which matches the shape of saidforming surface, said forming member and clamping structure beingpositionable relative to each other to form a space between said formingand pressure surfaces to accommodate grid wire, and the coefficients ofexpansion of said forming member and clamping structure being such thatsaid space tends to become smaller as said forming member and clampingstructure are heated.

7. Apparatus for making grid-like elements comprising a rod like mandrelhaving an upper. grooved portion and a lower shaft portion, and a jighaving a base portion with a bore slideably receiving said shaft portionof the mandrel, and support means extending up from said base to form asupporting surface around the mandrel.

References (Iited by the Examiner UNITED STATES PATENTS 1,974,312 9/1934Murphy -715 2,061,759 11/1936 Eitel et a1 140-715 X 2,135,288 11/1938Koenig 29-25.19 2,359,514 10/1944 Eitel et al. 140-715 2,398,609 4/1946Werner 140-715 IX 2,438,113 3/1948 Denmark 140-715 2,621,303 12/1952 Law313-348 2,877,805 3/1959 Van Meurs et a]. 140-715 2,921,363 1/1960Nielsen 29-2514 2,946,915 7/ 1960 Haase 313-348 FOREIGN PATENTS 392,51011/1908 France.

CHARLES W. LANHAM, Primary Examiner. RALPH G. NILSON, JAMES D. KALLAM,Examiners.

J. E. BECK, A. S. KATZ, L. A. LARSON,

Assistant Examiners.

1. A METHOD OF MAKING A GRID-LIKE ELEMENT FOR ELECTRON TUBES COMPRISINGTHE STEPS OF BENDING WIRE ONTO A FORMING MEMBER TO MAKE A DESIRED SHAPE,PLACING A SUPPORT MEMBER IN CONTACT WITH THE FORMED GRID WIRE,POSITIONING A CLAMPING STRUCTURE AROUND SAID FORMING MEMBER IN CONTACTWITH THE FORMED GRID WIRE, SAID CLAMPING STRUCTURE HAVING A LOWERCOEFFICIENT OF EXPANSION THAN SAID FORMING MEMBER, AND BY THEAPPLICATION OF HEAT SIMULTANEOUSLY BRAZING SAID SUPPORT MEMBER TO THEFORMED GRID WIRE AND COMPRESSING THE WIRE BETWEEN THE FORMING MEMBER ANDCLAMPING STRUCTURE.